In the field of gas turbine engines it is known that thrust is generated by combustion of a pressurized fuel and air mixture within a combustor of an engine core and discharge of the expanded, combusted fuel air mixture through a high-pressure and low-pressure turbine and out of the core engine through a variable discharge exhaust nozzle. Thrust is also generated by a rotating fan at an entry end of the engine. The fan is rotated, such as by the low pressure turbine or compressor, and by-pass air passing downstream from the fan is directed through a by-pass fan air duct that surrounds the engine core and discharges the by-pass air into the core engine air within the exhaust nozzle.
When a greatest amount of thrust is required, such as at take-off and ascent to a cruising altitude it is known that maximizing a fuel-air mixture within the combustors will produce the greatest amount of thrust. During such a take-off and ascent, it is known that by-pass air may be directed down a fan duct to cool the by-pass air duct and then exit into the core engine. However, at cruise altitude, less thrust is required. Therefore, to conserve fuel, the amount of fuel-air mixture delivered to the combustor is decreased and the by-pass fan air is increased and is directed to flow completely through the by-pass fan air duct so that a greater amount of thrust is contributed by the rotating fan in such a by-pass turbo fan gas turbine engine. Many problems are associated with varying flow of by-pass fan air into the core engine.
For example, U.S. Pat. No. 5,136,840 that issued on Aug. 11, 1992 to Nash shows channeling of by-pass air from a by-pass duct into an engine augmentor (or afterburner) through slots located at the upstream end of the augmentor. Varying the amount of air directed from the by-pass duct is performed by a complex actuation system that utilizes pivot arms and a ring sleeve with corresponding slots. This approach produces a substantial decrease in available thrust from both the fan and augmentor by having the by-pass air turn at ninety-degrees from within the by-pass duct into the augmentor, thereby disrupting linear flow of the exhaust stream which results in a pressure drop in both the by-pass air and core fuel air mixture within the augmentor. Additionally, substantial thrust may be lost from the fan air stream because complex actuators partially within the duct are required to rotate the sleeve valve. The same problems arise in U.S. Pat. No. 5,307,624 that issued on May 3, 1994 to Even-Nur et al., wherein a different actuator is utilized as part of a valve to direct by-pass air from a by-pass duct into an entry of an augmentor.
More recently, U.S. Pat. No. 8,181,441 that issued on May 22, 2012 to Smith (and that is owned by the owner of all rights in the present disclosure, and is incorporated herein by reference thereto) discloses a circumferentially rotating liner defining at least one aperture and disposed to rotate about a compressor section of a core engine for selectively admitting variable amounts of by-pass air from the by-pass duct through the aperture and into the core engine. This disclosure also produces substantial pressure drops within both of the by-pass air duct and core engine by having the by-pass air directed through an aperture to enter the core engine flow at a direction essentially normal to the core engine flow. Additionally, the sleeve valve actuator produces a pressure drop within the by-pass duct flow. Even more recently, U.S. Pat. No. 8,235,646 that issued on Aug. 7, 2012 to Lace discloses a variable area nozzle that changes its air passage area by extension and retraction into a by-pass fan duct of a fairing having one end slidably secured to an inner wall of the by-pass fan air duct and having an opposed end beneath the inner wall and actuated by a complex actuating mechanism. The fairing structure of Lace produces a very substantial loss of thrust in the by-pass fan duct, and involves a high degree of complexity in the actuating mechanism.
Therefore, there is a need for an improved, efficient gas turbine engine system for varying proportions of flow of by-pass fan air through a by-pass fan air duct and an engine core, and which minimizes pressure losses in mixing the air streams.